An electric power steering apparatus (EPS) which provides a steering system of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies a motor driving force as the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism, and performs assist control. In order to accurately generate the assist torque, such a conventional electric power steering apparatus performs feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of a duty ratio of pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft, a handle shaft) 2 connected to a steering wheel 1 is connected to steered wheels 8L and 8R through reduction gears (worm gears) 3 constituting the reduction mechanism, universal joints 4a and 4b, a rack and pinion mechanism 5, tie rods 6a and 6b, further via hub units 7a and 7b. In addition, a torsion bar is interposed in the column shaft 2, the column shaft 2 is provided with a steering angle sensor 14 for detecting a steering angel 6 of the steering wheel 1 in accordance with a twist angle of the torsion bar and a torque sensor 10 for detecting a steering torque Th, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist control command based on the steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 based on a voltage control command value Vref obtained by performing compensation and so on with respect to the current command value.
Further, the steering angle sensor 14 is not indispensable and may not be provided, and it is possible to obtain the steering angle from a rotational angle sensor such as a resolver connected to the motor 20.
A controller area network (CAN) 40 to exchanging various information of a vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vel from the CAN 40. Further, it is also possible to connect a non-CAN 41 exchanging a communication, analog/digital signals, a radio wave or the like except with the CAN 40 to the control unit 30.
The control unit 30 mainly comprises a CPU (including an MPU, an MCU and so on), and general functions performed by programs within the CPU are shown in FIG. 2.
The control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Th detected by the torque sensor 10 and the vehicle speed Vel detected by the vehicle speed sensor 12 (or from the CAN 40) are inputted into a current command value calculating section 31 that calculates a current command value Iref1. The current command value calculating section 31 calculates the current command value Iref1 that is a control target value of a current supplied to the motor 20 based on the steering torque Th and the vehicle speed Vel that have been inputted and by means of an assist map or the like. The current command value Iref1 is inputted into a current limiting section 33 through an adding section 32A. A current command value Irefm the maximum current of which is limited is inputted into a subtracting section 32B, and a deviation I (=Irefm−Im) between the current command value Irefm and a motor current value Im being fed back is calculated. The deviation I is inputted into a proportional integral (PI) control section 35 for improving a characteristic of the steering operation. The voltage control command value Vref whose characteristic is improved by the PI-control section 35 is inputted into a PWM-control section 36. Furthermore, the motor 20 is PWM-driven through an inverter 37. The motor current Im of the motor 20 is detected by a motor current detector 38 and is fed back to the subtracting section 32B. The inverter 37 is comprised of a bridge circuit of field effect transistors (FETs) as semiconductor switching elements.
A rotational angle sensor 21 such as a resolver is connected to the motor 20, and a rotational angle θ is detected and outputted by the rotational angle sensor 21.
A compensation signal CM from a compensation signal generating section 34 is added to the adding section 32A, and a characteristic compensation of the steering system is performed by the addition of the compensation signal CM so as to improve a convergence, an inertia characteristic and so on. The compensation signal generating section 34 adds a self-aligning torque (SAT) 343 and an inertia 342 at an adding section 344, further adds the result of addition performed at the adding section 344 with a convergence 341 at an adding section 345, and then outputs the result of addition performed at the adding section 345 as the compensation signal CM.
In the case that the motor 20 is a three-phase brushless motor, details of the PWM-control section 36 and the inverter 37 have a configuration as shown in FIG. 3, and the PWM-control section 36 comprises a duty calculating section 36A that calculates PWM duty values D1 to D6 for three phases by using the voltage control command value Vref in accordance with a predetermined expression, and a gate driving section 36B that drives the gates of the FETs serving as driving elements by means of the PWM duty values D1 to D6 and turns the gates on or off with compensating a dead time. The inverter 37 is configured to three-phase bridges of FETs (FET1 to FET6) serving as semiconductor switching elements, and drives the motor 20 by the three-phase bridges of the FETs being made turned on or off by means of the PWM duty values D1 to D6. A motor relay 39 for supplying (ON) or interrupting (OFF) electric power is connected to a power supply line between the inverter 37 and the motor 20 by the phase.
In such an electric power steering apparatus, a large current can flow in a motor in accordance with a steering situation (for example, a case where a steering wheel keeps hitting an end and being locked for a long time in a static steering state). When a coil in the motor has a high temperature, for example, more than or equal to 180 degrees Celsius, a problem of damage of the coil or the like occurs. Therefore, it is necessary to take measures not to overheat the coil from the viewpoint of safety of a vehicle, and to do so, it is necessary to estimate or measure a temperature of the coil (a coil temperature). However, since it is difficult to measure the coil temperature directly, methods to estimate the coil temperature have been proposed.
For example, the publication of Japanese Patent No. 5211618 B2 (Patent Document 1) constructs a temperature estimation model considering a relationship between heat transfer phenomena between poly-phase coils and a motor rotational velocity, and a relationship between a radiation coefficient and the motor rotational velocity, and estimates the coil temperature. Specifically, Patent Document 1 identifies heat transfer coefficients between a coil of any phase in a poly-phase motor and open air environment and between any phase and another phase in accordance with a change of the motor rotational velocity, and estimates a temperature of a coil of each phase or a magnet in the motor by using a substrate temperature and a current (or current command value) of each phase. The publication of Japanese Patent No. 4483298 B2 (Patent Document 2) estimates a temperature of a motor coil by utilizing that a calorific value of a motor is proportional to an integrated value of a square value of a current passing through the motor coil and that a temperature change of the motor coil affected by radiation (refrigeration) of the motor coil has a relationship of a primary delay function in a practically applicable temperature range (−40 to 180 degrees Celsius). Specifically, Patent Document 2 estimates the temperature of the motor coil by averaging a value obtained by squaring and integrating a value of the current passing through the motor coil, and making the result pass the primary delay function twice.
The List of Prior Art Documents Patent Documents    Patent Document 1: Japanese Patent No. 5211618 B2    Patent Document 2: Japanese Patent No. 4483298 B2